The Downlink Performance for Cell-Free Massive MIMO with Instantaneous CSI in Slowly Time-Varying Channels

Abstract

In centralized massive multiple-input multiple-output (MIMO) systems, the channel hardening phenomenon can occur, in which the channel behaves as almost fully deterministic as the number of antennas increases. Nevertheless, in a cell-free massive MIMO system, the channel is less deterministic. In this paper, we propose using instantaneous channel state information (CSI) instead of statistical CSI to obtain the power control coefficient in cell-free massive MIMO. Access points (APs) and user equipment (UE) have sufficient time to obtain instantaneous CSI in a slowly time-varying channel environment. We derive the achievable downlink rate under instantaneous CSI for frequency division duplex (FDD) cell-free massive MIMO systems and apply the results to the power control coefficients. For FDD systems, quantized channel coefficients are proposed to reduce feedback overhead. The simulation results show that the spectral efficiency performance when using instantaneous CSI is approximately three times higher than that achieved using statistical CSI.

Energy Efficiency of User-Centric, Cell-Free Massive MIMO-OFDM with Instantaneous CSI

“…The channel coefficients are estimated by the pilot-aided-based minimum mean squared error (MMSE) with overhead greater than K . According to the analysis in the literature, the downlink spectral efficiency (SE) of the n -th subchannel for UE k is as follows: where and indicates the variance of channel estimation error [ 27 ].…”

“…The user equipments (UEs) move more in line with the pedestrian model when the cell-free massive MIMO systems are deployed on campuses, in large venues, and in shopping malls. The literature analyzes that the downlink transmission rate of cell-free massive MIMO, using estimated instantaneous channel state information (CSI), can be substantially increased in slow-varying channels [ 27 ].…”